This invention relates to cationic polysaccharides which contain more than one cationic substituent per substituted hydroxyl group so as to provide a higher cationic charge density than that of known cationic polysaccharides This is achieved by a site selective reaction process, wherein a cationic polysaccharide derivative is first prepared employing a suitable cationic reagent, and then the pH is adjusted such that additional cationic reagent preferentially reacts with the cationic substituents rather than the remaining hydroxyl groups of the polysaccharide.
The modification of starch and other polysaccharides by chemical derivatization to produce various cationic polysaccharides is well known. Cationic polysaccharides, i.e., polysaccharides which have been modified so that they have a positive electrostatic charge, are used for a large number of applications and are particularly useful in the manufacture of paper due to their superior performance in the paper production as compared to unmodified polysaccharides. Amphoteric polysaccharides i.e., polysaccharides which have been modified so they have cationic groups, together with a controlled amount of anionic (e.g., phosphate) groups, are used in a similar manner, with superior performance as compared to unmodified polysaccharides.
As used herein, the term "paper" includes sheet-like masses and molded products made from fibrous cellulosic material, which may be derived from natural sources as well as from synthetics such as polyamides, polyesters and polyacrylic resins, as well as from mineral fibers such as asbestos and glass. Also included are papers made from combinations of cellulosic and synthetic materials.
Various materials, including starch, are added to the pulp or stock, during the paper-making process, prior to the formation of the sheet. One purpose of such additives is to bind the individual fibers to one another, thus aiding the formation of a stronger paper. Alum is employed in traditional paper-making processes which are conducted under acidic conditions. Alum-free, alkaline conditions in paper-making processes are now becoming common in the industry.
In the case of those papers which contain added pigments, such as titanium dioxide, it has been known to add materials to the pulp, or stock, for the specific purpose of retaining a greater proportion of such pigments in the paper (rather than have them drain off in the water that is removed during the formation of the sheet). Such additives are often referred to as "pigment retention agents." Cationic starches have long been employed as additives in paper production for their contributions to drainage, strength and pigment and fine pulp retention in paper.
It has now been discovered that unexpected superior performance in paper production may be achieved by the use of novel cationic polysaccharide derivatives which are prepared by reaction of a polysaccharide with a cationic reagent employing a two-stage, site selective reaction to produce a di- or multi-cationic, high charge density polysaccharide derivative. The performance of these derivatives far exceeds their expected performance based on molar substitution alone. Their performance is most advantageous in alum-free processes for making paper under alkaline conditions. It is believed that the high charge density per saccharide monomer unit is responsible for the unexpected improvement in performance.
Accordingly, this invention provides a new class of cationic polysaccharide derivatives prepared by a site selective reaction. These cationic polysaccharides are useful in paper manufacturing and show improved drainage, pigment and pulp retention and paper strength as compared to cationic polysaccharides of the prior art.